Fixed max_key_length when using UNIQUE keys.

This fixed a bug in GROUP BY on a BLOB column with NULL values.

myisam/mi_create.c

@@ -224,7 +224,7 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
   if (uniques)
   {
     max_key_block_length= MI_KEY_BLOCK_LENGTH;
-    max_key_length= MI_UNIQUE_HASH_LENGTH;
+    max_key_length= MI_UNIQUE_HASH_LENGTH + pointer;
   }
 
   for (i=0, keydef=keydefs ; i < keys ; i++ , keydef++)

myisam/mi_unique.c

@@ -24,7 +24,7 @@ my_bool mi_check_unique(MI_INFO *info, MI_UNIQUEDEF *def, byte *record,
 {
   my_off_t lastpos=info->lastpos;
   MI_KEYDEF *key= &info->s->keyinfo[def->key];
-  uchar *key_buff=info->lastkey+info->s->base.max_key_length;
+  uchar *key_buff=info->lastkey2;
   DBUG_ENTER("mi_check_unique");
 
   mi_unique_store(record+key->seg->start, unique_hash);
@@ -80,8 +80,17 @@ ha_checksum mi_unique_hash(MI_UNIQUEDEF *def, const byte *record)
     if (keyseg->null_bit)
     {
       if (record[keyseg->null_pos] & keyseg->null_bit)
+      {
+	/*
+	  Change crc in a way different from an empty string or 0.
+	  (This is an optimisation;  The code will work even if this isn't
+	  done)
+	*/
+	crc=((crc << 8) + 511+
+	     (crc >> (8*sizeof(ha_checksum)-8)));
 	continue;
       }
+    }
     pos= record+keyseg->start;
     if (keyseg->flag & HA_VAR_LENGTH)
     {

mysql-test/r/group_by.result

@@ -102,3 +102,11 @@ pid	c1id	c2id	value	id	active	id	active
 1	4	NULL	4	4	Yes	NULL	NULL
 max(value)
 4
+a	count(*)
+NULL	9
+	3
+b	1
+a	count(*)
+NULL	9
+	3
+b	1

mysql-test/t/group_by.test

@@ -312,3 +312,14 @@ m.c1id = c1.id AND c1.active = 'Yes' LEFT JOIN t3 AS c2 ON m.c2id =
 c2.id AND c2.active = 'Yes' WHERE m.pid=1  AND (c1.id IS NOT NULL OR c2.id IS 
 NOT NULL);
 drop table t1,t2,t3;
+
+#
+# Test bug in GROUP BY on BLOB that is NULL or empty
+#
+
+create table t1 (a blob null);
+insert into t1 values (NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(""),(""),(""),("b");
+select a,count(*) from t1 group by a;
+set option sql_big_tables=1;
+select a,count(*) from t1 group by a;
+drop table t1;